The 40-year-old actress said she felt the performance pressure while working with some of of Hollywood's finest actors, reported Female First.
"It was really intimidating I have to say. At the read through I was a bit like, 'Oh my gosh! I have to act with all of these incredible people.
"But they were so nice and so down to earth. Really, really lovely and so supportive," Harris said.
The actress said, the cast - which also includes Kate Winslet and Keira Knightley - were all really "supportive" and "down to earth".